1. Field of the Invention
The present invention relates to a semiconductor laser device having a BTlGaAs active layer on a GaAs substrate and operating in a long wavelength region for communication purposes.
2. Description of the Related Art
Optical communication generally employs signal light having wavelengths of 1.3 .mu.m and 1.55 .mu.m, and an InGaAsP/InP semiconductor laser device on an InP substrate is used as the light source.
However, the InGaAsP/InP semiconductor laser has a problem that the discontinuity in the conduction band between the InGaAsP active layer and InP clad layers which sandwich the active layer is as small as about 100 meV and, as a consequence, overflow of electrons from the active layer into the clad layer becomes conspicuous as the temperature rises, resulting in rapidly increasing current.
A GaAs/AlGaAs laser and a strained quantum well laser based on InGaAs/AlGaAs on a GaAs substrates, on the other hand, can provide better temperature characteristics because AlGaAs having a larger band gap energy can be used as the clad layer which produces a discontinuity in the conduction band as large as 300 meV or greater.
When producing a surface emitting laser with a perpendicular resonance, a GaAs/AlAs semiconductor multi-layer film can be used as a reflector layer, and therefore high reflectivity can be obtained.
It is difficult to make practical use of an active layer material which can be grown on a GaAs substrate while achieving lattice matching with the GaAs substrate and is capable of oscillating with a wavelength of 1.3 .mu.m or 1.55 .mu.m.
Researches on growing a semiconductor layer made of a GaInNAs material on a GaAs substrate have been done (for example, Kondo & Uomi; Applied Physics vol.65 (1996), p148), but commercial applications have not been achieved due to a problem in crystalline characteristics and other reasons.